Optical printing systems use output scanner systems wherein the intensity of a laser light beam focused on a moving two-dimensional photosensitive surface is modulated as the beam is line scanned relative to such moving surface to provide a two-dimensional output image. In one common output scanner system, a deflector, such as a rotating polygon mirror, line scans a beam of laser light. The intensity of such scanned light beam is modulated by an acoustooptic cell. Precise synchronization clock signals are necessary to represent the position of the laser beam as it is line scanned. The timing of the modulation of the laser beam is controlled by these clock signals. The clock signals control the flow of information from an electronic data buffer to the modulator. The acoustooptic modulator controls the amount of light. Light modulation can be accomplished at high frequencies of 6 MHz or more. One common technique used to provide clock signals is a grating clock. In this technique, a second unmodulated laser beam is also reflected off the rotating mirror surface and scans a grating that intensity modulates the second light beam. A mirror projects the intensity modulated second light beam onto the surface of a detector which provides the synchronization clock signals as the line scan progresses. This system offers a number of advantages in that the clock signals produced are representative of the instantaneous beam position. U.S. Pat. No. 3,835,249 to Dattilo et al, issued Sept. 10, 1974, discloses such a system. One problem with grating clock arrangements is that they require other optical elements such as a grating element that must be accurately aligned to produce the desired clock signals.
In another approach, start-of-scan (SOS) and end-of-scan (EOS) detectors are located near the image plane and produce signals when illuminated by the scanning light beam. Clock signals must be generated which are started in phase with the start-of-scan (SOS) signal.
One method which has been used at moderate clock rates (i.e. 5 MHz) is to connect a crystal oscillator signal which produces signals at eight times (40 MHz) the desired clock signal frequency to a divide-by-eight counter. The counter is held clear until the SOS pulse occurs. The first clock pulse signal occurs one-half clock period later. However, variations in the start of the first clock pulse can cause as much as one eighth of a pixel variation in corresponding pixels from line to line. The EOS signal is used to clear the counter. At high clock rates, this method becomes difficult to implement. For example, a 12 MHz clock signal would require a counter which is fast enough to accept a 96 MHz crystal clock signal.